With the development of radio and space technologies, several satellites based navigation systems (i.e. satellite positioning system or “SPS”) have already been built and more will be in use in the near future. SPS receivers, such as, for example, receivers using the Global Positioning System (“GPS”), also known as NAVSTAR, have become commonplace. Other examples of SPS systems include but are not limited to the United States (“U.S.”) Navy Navigation Satellite System (“NNSS”) (also known as TRANSIT), LORAN, Shoran, Decca, TACAN, NAVSTAR, the Russian counterpart to NAVSTAR known as the Global Navigation Satellite System (“GLONASS”) and any future Western European SPS such as the proposed “Galileo” program. As an example, the U.S. NAVSTAR GPS system is described in GPS Theory and Practice, Fifth ed., revised edition by Hofmann-Wellenhof, Lichtenegger and Collins, Springer-Verlag Wien NewYork, 2001, which is fully incorporated herein by reference.
The U.S. GPS system was built and is operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
A GPS receiver typically has to acquire and lock onto at least four satellite signals in order to derive the position and time. Usually, a GPS receiver has many parallel channels with each channel receiving signals from one visible GPS satellite. The acquisition of the satellite signals involves a two-dimensional search of carrier frequency and the pseudo-random number (PRN) code phase. Each satellite transmits signals using a unique 1023-chip long PRN code, which repeats every millisecond. The receiver locally generates a replica carrier to wipe off residue carrier frequency and a replica PRN code sequence to correlate with the digitized received satellite signal sequence. During the acquisition stage, the code phase search step is a half-chip for most navigational satellite signal receivers. Thus the full search range of code phase includes 2046 candidate code phases spaced by a half-chip interval. The carrier frequency search range depends upon the Doppler frequency due to relative motion between the satellite and the receiver. Additional frequency variation may result from local oscillator instability.
The signals from the navigational satellites are modulated with navigational data at 50 bits/second (i.e., 1 bit/20 msec). This navigational data consists of ephemeris, almanac, time information, clock and other correction coefficients. This data stream is formatted as sub-frames, frames and super-frames. A sub-frame consists of 300 bits of data and is transmitted for 6 seconds. In this sub-frame a group of 30 bits forms a word with the last six bits being the parity check bits. As a result, a sub-frame consists of 10 words. A frame of data consists of five sub-frames transmitted over 30 seconds. A super-frame consists of 25 frames sequentially transmitted over 12.5 minutes.
The first word of a sub-frame is always the same and is known as TLM word and first eight bits of this TLM word are preamble bits used for frame synchronization. A Barker sequence is used as the preamble because of its excellent correlation properties. The other bits of this first word contain telemetry bits and are not used in the position computation. The second word of any frame is the HOW (Hand Over Word) word and consists of TOW (Time Of Week), sub-frame ID, synchronization flag and parity with the last two bits of parity always being ‘0’s. These two ‘0’s help in identifying the correct polarity of the navigation data bits. The words 3 to 10 of the first sub-frame contains clock correction coefficients and satellite quality indicators. The 3 to 10 words of the sub-frames 2 and 3 contain ephemeris. These ephemeris are used to precisely determine the position of the GPS satellites. These ephemeris are uploaded every two hours and are valid for four hours to six hours. The 3 to 10 words of the sub-frame 4 contain ionosphere and UTC time corrections and almanac of satellites 25 to 32. These almanacs are similar to the ephemeris but give a less accurate position of the satellites and are valid for six days. The 3 to 10 words of the sub-frame 5 contain only the almanacs of different satellites in different frames. The super frame contains twenty five consecutive frames. While the contents of the sub-frames 1, 2 and 3 repeat in every frame of a superframe except the TOW and occasional change of ephemeris every two hours. Thus the ephemeris of a particular signal from a satellite contains only the ephemeris of that satellite repeating in every sub-frame. However, almanacs of different satellites are broadcast in-turn in different frames of the navigation data signal of a given satellite. Thus the 25 frames transmit the almanac of all the 24 satellites in the sub-frame 5. Any additional spare satellite almanac is included in the sub-frame 4.
The almanac and ephemeris are used in the computation of the position of the satellites at a given time. The almanacs are valid for a longer period of six days but provide a less accurate satellite position and Doppler compared to ephemeris. Therefore, almanacs are not used when a fast position fix is required. On the other hand, the accuracy of the computed receiver position depends upon the accuracy of the satellite positions which in-turn depends upon the age of the ephemeris. The use of current ephemeris results in better and faster position estimation than one based on non-current or obsolete ephemeris. Therefore, it is necessary to use current ephemeris to get a fast receiver position fix.
A GPS receiver may acquire the signals and estimate the position depending upon the already available information. In the ‘hot start’ mode the receiver has current ephemeris and the position and time are known. In another mode known as ‘warm start’ the receiver has non-current ephemeris but the initial position and time are known as accurately as in the case of previous ‘hot start’. In the third mode, known as ‘cold start’, the receiver has no knowledge of position, time or ephemeris. As expected the ‘hot start’ mode results in low Time-To-First-Fix (TTFF) while the ‘warm start’ mode which has non-current ephemeris may use that ephemeris or the almanac resulting in longer TTFF due to the less accurate Doppler estimation and ephemeris downloading. The ‘cold start’ takes still more time for the first position fix as there is no data available to aid signal acquisition and position fix.
Therefore, it is necessary to keep the ephemeris in the receiver current for a fast TTFF. Current ephemeris also helps when the received signal is weak and the ephemeris can not be downloaded. Some issued patents teach receiving the ephemeris through an aiding network or remote server instead of from an orbiting satellite. However, this approach results in higher cost and requires additional infrastructure. Another approach to keeping ephemeris current, without using a remote server, is to automatically download it from satellites in the background, such as described in U.S. Pat. No. 7,435,357.
Some commercially available products such as SiRF InstantFixII from SiRF Technologies of San Jose, Calif. use extended ephemeris to improve start-up times without requiring network connectivity. With one observation of each satellite, SiRFInstantFixII accurately predicts satellite positions for up to three days—removing the need to download satellite ephemeris data at subsequent start-ups—resulting in full navigation in as little as five seconds, and with routine 7 meter accuracy. Moreover, such extended ephemeris products not only start tracking satellites and navigating more quickly, they can do it using signals much weaker than those needed to obtain satellite location data the traditional way, removing the barrier that often blocks successful navigation under tough GPS signal conditions.
When a GPS signal is not available and a cellular signal is available, others have used the cellular signal to update the ephemeris data and time information. However, this method leads to additional cellular over the air charges.
Nevertheless, some challenges remain. For example, when the receiver is in a location in which satellite signal is not available such as large office buildings, subways, indoor malls, expo halls, etc., the GPS receiver's stored ephemeris data and time may become out-of-date. Without any GPS signal, cellular signal or other supplementary external data source, the tracker cannot determine when to start looking for SV or cellular signal. Under these circumstances the battery for the GPS receiver can quickly be run down by repeated unsuccessful attempts to update the ephemeris and time data.